As demand for crude oil and other fossils fuels has increased, the use of undersea drilling operations has expanded. Such operations, however, present an environmental risk in that catastrophic failure in drilling equipment could result in dangerous emission of oil leaking into the undersea environment. As a safety mechanism, undersea drilling operations typically are equipped with blowout preventers. In the event of a catastrophic failure of drilling equipment, the blowout preventers are triggered, which incorporate wedge-like blocking components that drive into the drill stream to cut off flow, thereby preventing leakage into the environment.
The blowout preventers typically are powered using high pressure accumulators, with the flow of the hydraulic fluid being controlled by valve systems. Such valve systems typically may employ hydraulic shuttle valves. A hydraulic shuttle valve generally includes a valve body having a bore with two fluid inlets, one fluid inlet each on opposite ends of the bore. There also is a fluid outlet perpendicular to the bore and between the two fluid inlets. A moveable member rides in the bore of the valve body, and shifts position between two stops or valve seats in the valve body based on the pressures that occur at the two fluid inlets. The movable member will shift when there is a pressure differential between the two fluid inlets to create a net force to shift the moveable member against one of the valve seats, depending upon the direction of the pressure differential. The movable member will reach a physical stop on either side of the bore when the moveable member comes in contact with one of the valve seats. Each valve seat may be integrated into an end cap, or constitute a separate component.
A shuttle valve generally is used as a junction between two separate upstream circuits and a single downstream function. In the case of a blowout preventer in an undersea drilling operation, for example, the two upstream circuits are separate control pods for sourcing hydraulic fluid that are required for redundancy in case of a failure of one of the circuits. Hydraulic fluid will flow in through one of the fluid inlets and then out through the common fluid outlet to actuate a downstream function, such as a large hydraulic cylinder or shear ram in order to seal off the well head and prevent a blowout.
When the hydraulic cylinder retracts, a condition arises where hydraulic fluid will be flowing in the reverse direction, in through the fluid outlet of the shuttle valve, and the reverse flow needs to evacuate through one of the two fluid inlets. During this condition, the shuttle valve must remain biased towards one of the valve seats, for if the movable member were to shift to an intermediate position, the flow area would be reduced resulting in a lower flow rate and ultimately a slower cylinder retraction time. In addition, an intermediate position can lead to an undesirable state of unstable “chatter”, in which the moveable member rapidly moves back-and-forth between the two valve seats. Chattering also reduces the reverse flow rate for cylinder retraction, and increases wear on the shuttle valve due to the repeated impacts of the moveable member against the valve seats.
Shuttle valve stabilization, therefore, is important to maintain an efficient bias state of the moveable member towards one of the valve seats during the reverse flow. Conventional configurations for achieving such bias have significant drawbacks. Certain solutions attempt to maintain the moveable member captive in one of the valves seats during the reverse flow. This typically requires that the reverse flow actually be forced through the moveable member itself, which minimizes flow area. Other conventional configurations have used o-rings or similar components to provide friction to hold the moveable member toward the valve seats. Such configurations, however, restrict the movement of the moveable member generally, thereby undesirably increasing the requisite operating flows and pressures. Such configurations further add a component to the valve system, which in some cases can fail resulting in undesired and potentially catastrophic system contamination.
Conventional shuttles valves have thus proven deficient in providing a maximum reverse flow rate for hydraulic cylinder retraction, while preventing the centering of the moveable member in an intermediate state and unstable chattering.